How can I change the name of a second hand MacBook Pro?

here is the apple support document: Change the name of your macOS user account and home folder - Apple Support. you need to follow those instructions TO THE LETTER. even one missed step can result in getting locked out of the Mac. it can be really difficult to recover from that. and you should make a backup first incase something goes sideways when changing the name of the account.

IMHO, it is much easier to create a NEW admin account on the Mac, and then simply remove the old account from the Mac. please see Set up users, guests, and groups on Mac.

and if there is data in the old account that you want / need, transfer that data to an external drive BEFORE starting.

DON'T try to change that name.

Far too many users end up back here crying that they are permanently locked out of their computer and all their files are lost after they tried.

Just create a NEW admin user. Log in as that User, and use that name going forward.

If you need something from the old user account, it is next to yours in the /Users folder. Just navigate over there and copy what you what from the old User account.

after month, if you have not gone fishing in the Old user account, you can delete it and NOT save its files.

The "Approved Method" for dealing with a second-hand computer involves erasing it and re-installing the "shipped in the box" version of macOS automatically. This is what the seller was supposed to do before sending the computer to you:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Grant Bennet-Alder wrote:

The "Approved Method" for dealing with a second-hand computer involves erasing it and re-installing the "shipped in the box" version of macOS automatically. This is what the seller was supposed to do before sending the computer to you:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

I highly recommend @Mariam_shaf3y start with Step #6 in that Apple article to perform a clean install of macOS.

I think everyone should do a clean install whenever they acquire a used Mac even if the previous owner has done so, just to make sure it is done properly. Plus it can sometimes reveal hidden problems such as the Mac being firmware locked or still controlled by a previous owner.

Just creating a new macOS admin user account leaves the new owner open to a lot of potential problems & issues. Who knows the actual condition of the current macOS installation, or whether the previous owner intentionally (or accidentally) left behind some nefarious surprises.